Conventional compression ignition engines, such as diesel engines, produce less gaseous hydrocarbon (HC) and carbon monoxide (CO) than gasoline engines and it is possible to meet present legislated limits for these components using a platinum (Pt)-based diesel oxidation catalyst (DOC) disposed on a flow through honeycomb monolith. Diesel nitrogen oxide (NOx) emissions are presently controlled by engine management, such as exhaust gas recirculation (EGR). As a consequence, however, diesel particulate matter (PM) emissions including volatile and soluble organic fractions (VOF and SOF respectively) of unburned hydrocarbons (HC) are increased. The DOC is used to treat VOF and SOF in order to meet presently legislated limits for PM.
However, as emission standards are tightened in forthcoming years, the challenge of the skilled engineer is how to meet them.
Devices for treating exhaust gases from compression ignition engines such as diesel engines to meet present and future emissions standards include the DOC, the CRT®, catalysed soot filters (CSF), NOx traps, lean NOx catalysts (LNC) (also known as hydrocarbon selective catalytic reduction (HC-SCR) catalysts or non-selective catalytic reduction (NSCR) catalysts) and selective catalytic reduction (SCR) catalysts, i.e. using NOx-specific reactants such as ammonia or ammonia precursors e.g. urea.
An illustrative DOC composition for treating CO, HC and a VOF component of particulates in diesel exhaust is disclosed in WO 94/22564, which catalyst comprising ceria and a zeolite and optionally alumina carrying an optional dispersed metal component of Pt or palladium (Pd). Alternatively, or additionally, the zeolite is optionally doped, e.g. by ion-exchange, with inter alia Pt and/or Pd.
In our EP 0341832 we disclose a process for combusting diesel particulate deposited on a filter in nitrogen dioxide (NO2) at up to 400° C., which NO2 is obtained by oxidising nitrogen monoxide (NO) in the exhaust gas over a suitable catalyst disposed upstream of the filter. The NO oxidation catalyst can comprise a platinum group metal (PGM) such as Pt, Pd, ruthenium (Ru), rhodium (Rh) or combinations thereof, particularly Pt. The filter can be coated with material which facilitates higher temperature combustion such as a base metal catalyst, e.g. vanadium oxide, La/Cs/V2O5 or a precious metal catalyst. Such a system is marketed by Johnson Matthey as the CRT®.
WO 00/29726 discloses an apparatus for treating an exhaust gas stream, including diesel engine exhaust, which apparatus comprising a CSF comprising a first catalyst and a second catalyst in communication with the first catalyst. The first catalyst can comprise at least one first PGM including mixtures of PGM components; a first cerium component; and preferably a zirconium component. The second catalyst can comprise a second cerium component and optionally at least one second PGM. The second catalyst can be a separate catalytic element or part of the filter and is preferably designed for reducing diesel exhaust particulates emission by oxidation of the VOF thereof. None of the Examples describe a first or second catalyst comprising Pd.
A method of absorbing NOx from lean internal combustion engine exhaust gas on a NOx absorbent and intermittently reducing the oxygen concentration in the exhaust gas to release the absorbed NOx for reduction over a suitable catalyst with a reductant, thereby regenerating the NOx absorbent, is disclosed in EP 0560991.
A problem with such devices is that when the exhaust gas temperature is too low e.g. during extensive periods of idling or slow driving conditions, the catalysts in the devices are sub-optimally active. Consequently, emissions of legislated pollutants such as CO, HC and NOx increase and filters become loaded with PM. For example in the case of NOx traps, the NO oxidation catalyst has to be sufficiently hot that it can oxidise NO to NO2 so that the NO2 can be absorbed on a suitable NOx absorbent. In the use of CSF, the filter can be regenerated actively by combusting injected HC fuel thereon in order to raise the filter temperature to about 600° C. However, unless the filter is above about 250-300° C. prior to HC injection, the HC may not be combusted on the filter or combustion may be incomplete, thus leading to increased HC and CO emissions.
Measures for increasing the temperature in a system comprising a CRT® and a NOx trap are disclosed in EP 0758713.
A problem with prior art measures for increasing temperature in exhaust systems comprising DOC, the CRT®, CSF or NOx traps is that generally they result in an increased fuel penalty.
Two ways of reducing diesel emissions, which can be used in addition to exhaust gas aftertreatment, are engine management and engine design. More recently, a new generation of compression engines has been developed which uses a range of engine management techniques to lower the combustion temperature. Broadly, this new generation of engine can be defined as “an engine with compression ignition wherein substantially all of the fuel for combustion is injected into a combustion chamber prior to the start of combustion”. An exhaust system for treating exhaust gas from such engines is the subject of a related application to the present application filed on the same date entitled “Process for treating compression ignition engine exhaust gas” claiming an earliest priority date of 13th Sep. 2002. For the avoidance of doubt, the present application does not embrace the new generation of compression ignition engines as defined hereinabove.
Our EP 0602865 discloses a catalyst for oxidising CO to CO2 in the exhaust gas of an internal combustion engine, which catalyst is composed of metal oxide particles among which are uniformly incorporated noble metal particles obtainable e.g. by co-precipitation. The metal oxide particles can be CeO2 and the noble metal can be one or more of Pt, Pd, Rh and gold (Au).
Our WO 96/39576 discloses an internal combustion engine, such as a diesel engine, comprising an exhaust system comprising inter alia the CO oxidation catalyst disclosed in EP 0602865 for generating an exotherm from CO oxidation to light-off a HC oxidation catalyst following cold start. The engine is configured to produce increased levels of CO in the exhaust gas following cold start and the exhaust system preferably includes one or more of the following features for decreasing the CO light-off temperature: an HC trap and/or a water trap upstream of the CO oxidation catalyst; a water trap downstream of the CO oxidation catalyst; and CO catalyst drying means, such as a pump for passing dried ambient air over the CO oxidation catalyst prior to start-up.
DE 4117364 discloses a catalyst featuring an ancillary catalyst upstream of a main catalyst for lighting-off the main catalyst following cold start. The main catalyst is a 5Pt/1Rh three-way catalyst for treating stoichiometric gasoline exhaust gas. The ancillary catalyst is preferably Pt “which is outstanding for the oxidation of CO”, but can also be the more expensive 5Pt/1Rh catalyst or Pd but “certainly Pd is less active than Pt”.
JP 5-59937 describes a system for treating start-up exhaust gases from a gasoline engine including an HC trap upstream of a catalyst for oxidising CO for heating up a downstream exhaust purifying catalyst in a start-up strategy. The CO oxidation catalyst can be 0.5% Pd/Al2O3 which can be co-existent with the exhaust purifying catalyst, coated on an upstream side of a brick having the exhaust purifying catalyst on the downstream end or layered with the exhaust purifying catalyst. Engine management provides 6% peak CO at cold start falling to 1% CO after 20 seconds, but optionally can be kept at 3% CO until the exhaust purifying catalyst has warmed up, as necessary.
By “metal” herein, we mean the oxidic compound existing in the presence of the constituents of exhaust gas, although in use they may be present as the nitrate, carbonate or hydroxide.
We have investigated Pd catalysts for CO oxidation and have found that Pd catalysts are at least of zero order kinetics for CO for the reaction, i.e. the rate of reaction stays the same regardless of the CO concentration. We have also found that for certain promoted and supported Pd catalysts, the rate of reaction is first order for CO, i.e. the more CO, the faster the rate of reaction. By contrast, a widely used PGM in DOCs, platinum (Pt), can be negative order in CO, i.e. the more CO, the lower the reaction rate.
Furthermore, in tests we have found that our supported and promoted Pd catalysts can be better than Pt at catalysing the oxidation of certain saturated HCs.
We have now found a way of utilising our observations in an exhaust system of a compression ignition engine, such as a diesel engine, to further reduce overall emissions.